The present invention relates in general to crystal filters and, more particularly, to an improved monolithic crystal filter device which eliminates transmission zeros giving rise to undesirable flyback and provides a significant improvement in the ultimate attenuation of the filter device.
Bandpass filter circuits comprising monolithic crystal filter elements have been used frequently in the past since they provide a high degree of selectivity in a relatively compact arrangement. Such monolithic crystal filters have found particularly advantageous use in the front end circuitry of small, hand-held radio receivers where high selectivity is required but space is at a premium.
A problem, however, in monolithic crystal filters heretofore has been that a significant level of capacity exists between input and output electrodes giving rise to transmission zeros and, consequently, undesirable fly back in the attenuation characteristic thereof. In the past, some reduction in the inherent capacity has been realized by the expedient of reversing one pair of electrodes with respect to the other. That is, by including input and output electrodes on opposing sides of the quartz wafer blank. This, in turn, results in the respective ground electrodes likewise being positioned on alternate sides of the crystal blank. The resulting configuration, while permitting a modest improvement of a few db, nevertheless leaves a good deal to be desired and does not approach desired operating level consonant with modern day technology.
Accordingly, it is an object of the present invention to provide a monolithic crystal filter device with improved overall attenuation characteristics.
A more particular object of the present invention is to provide an improved two-pole monolithic crystal filter device exhibiting a substantial improvement in flyback and ultimate attenuation level of the filter device.
Another object of the present invention is to provide an improved monolithic crystal filter having input and output electrodes which are substantially shielded from one another so as to reduce inherent capacity therebetween and to effect the desired improvement in operating characteristics.
Still another object of the present invention is to provide an improved monolithic crystal filter where not only are the input and output electrodes dimensioned and positioned on the crystal blank to optimize operating characteristics, but wherein the ground electrodes are utilized to play a significant role in such optimized operating levels.
In practicing the invention, a monolithic crystal filter device is provided having a pair of electrodes formed on a single quartz crystal wafer. The electrode pairs, in conjunction with the piezoelectric material of the wafer, form two resonating portions.
Each electrode pair comprises an input or output electrode, as the case may be, in conjunction with an associated ground electrode. The active electrodes are positioned on opposing sides of the quartz wafer end to end with a central gap therebetween. The ground electrodes are likewise positioned on opposing sides of the quartz wafer along an axis essentially perpendicular to the axis of the active electrodes. Further, the ground electrodes are positioned to overlie an end portion of its respective input or output electrode and to extend into the central gap separating the in-board end of the respective active electrodes. In the preferred embodiment, the ground electrodes extend into the interelectrode gap to an extent whereby respective portions on the opposing sides of the wafer blank overlie one another.